CDL Air Brakes TestAir Brake System PartsMedium
A driver is inspecting a commercial vehicle and finds that the front brake limiting valve is set to 'slippery'. What is the purpose of this setting in an air brake system?
- ATo decrease air pressure to the front brakes to prevent front wheel lock-up.
- BTo activate the anti-lock braking system (ABS) on the front axle.
- CTo disable the front brakes entirely in adverse conditions.
- DTo increase air pressure to the front brakes for better stopping power.
Show answer & explanationAnswer & explanation
Correct answer: A. To decrease air pressure to the front brakes to prevent front wheel lock-up.
The front brake limiting valve, when set to 'slippery' (often 1/2 power), reduces the air pressure applied to the front brakes by half. This reduces the risk of front wheel lock-up on slippery surfaces, which can lead to loss of steering control.
Why the other options are wrong
- B. The limiting valve is a mechanical control, separate from the electronic ABS, though both aim to prevent lock-up.
- C. The valve limits pressure, it does not disable the brakes entirely.
- D. This setting reduces pressure, not increases it.
Front Brake Limiting Valve
A mechanical valve that allows the driver to reduce the air pressure to the front brakes, typically by half, to prevent front wheel lock-up on slippery road surfaces.
- Reduces front brake pressure by half in 'slippery' mode.
- Prevents front wheel lock-up and loss of steering.
- Found on older vehicles; ABS largely replaces its function.
Memory trick: Slippery switch, Slows the front, for Steering security.